Services

Simple, don't leave the unit unattended. 

If there are 2 of you, visit the facilities one at a time, or use your own. You can even make your own snacks and drinks too.

If it becomes essential to leave the van unattended, use your hitch lock to lock it to the car's towball but never drive with it locked on.

You do hear stories like what you suggest and we have never left a caravan or motorhome unattended at a motorway services. I usually stay with the van as Margaret goes off to get the coffee and anything we might want to eat. Whilst I am with the van I often see people leave their units and wander off to the services. When I notice that I tend to keep a watching brief and have never witnessed anything unusual but it does happen. Just seen a post on Facebook about a motorhome being stolen from someone's property where the thieves actually shunted parked cars out of the way in order to give them access to the motorhome so anything is possible. If its just a comfort stop why not use the facilities in the van?

David

Parking up can be a bit of a nightmare, as few motorway services have dedicated caravan/motorhome areas, and if they do, they are generally busy. Suggest doing some research prior to each journey, until you get to know the stops. You are often in with the HGVs at some services, so be aware when stepping out of vans and take extra precautions for children and pets. Scotch Corner at the A1/A66 junction does not allow caravans, it’s simply too tight. Tinny has offered very good security pointer.

If you look at your whole journey, take note of any none motorway route, and see if there is a supermarket, or lay by on it. It can often be far easier, cheaper and relaxing, using your onboard facilities, than stopping off at services.

You can use Google maps street view to 'drive' into the services to get an idea of where the caravan/motorhome parking is. If in doubt use the lorry park, there is usually space there.

I always put my hitch lock on the caravan when using the services and, if not taking the coffee back to the car, try to sit so that I can see the unit whilst drinking.

We just lock our motorhome up and go into the services,  like we would with our car. If it looked a dodgy area would probably move on. If leaving it parked for a wee bit longer I would put the disclok on.

I searched for any information on thefts from services and could find none. All the articles about caravan and motorhome thefts were about them being stolen from home and usually to order.

I wonder if this is a myth about high level of thefts from services? It would have to be opportunistic and unless it was when the car park was quiet there are likely to be plenty of witnesses.

A slightly different tack - Services, unfortunately, are a necessity when traveling a distance and being unsure of an alternative, try to pick those who have a separate area for caravans. The stench of urine around the areas where the lorries park is disgusting! Trying to get the dog out for her comfort break and then sitting having something to eat with the car isn’t pleasant 🙈 We’ve been more inclined to use the Toll near Birmingham when going south 😂

I don't think it is a myth but, regardless, it makes,sense to take security precautions.

The OP has a caravan and they are more vulnerable to being stolen than motorhomes.

We left caravans at services, home and away, for over 50 years without any problems - and at supermarkets too. We even left them unattended on caravan sites as well. 

Just an alternative to consider - if you are towing a long distance,  why not break your journey by stopping overnight at a CL en route? Much more secure and pleasant than stopping at services for a short break, you'll arrive at your final destination more relaxed and possibly discover the delights of parts of the country you might not otherwise consider visiting. 

There's a bit of a difference between a caravan site & motorway services